+1 to what Mr. Silver said. I certainly do think you could do an extra 10 miles - but, if you already know your bike is causing you some pain &/or discomfort, maybe get that checked out first.

If your neck and shoulders are getting sore, there's a number of things that could be causing it, and it might be useful to get a good bike fitter to help you figure out if you can tweak your current bike to be more comfortable.

FWIW, I made a couple changes that helped my neck/shoulder pain a little: I made sure my saddle was level (if it's angled down, you tend to put more stress on your upper body to keep from sliding forward), and I got rid of the visor on my helmet (with the visor, I had to tilt my head further up to see down the road; without it I can keep my head in a more neutral position and just move my eyes). I don't know if either of these are a factor for you; it may be that you would need to actually replace parts on your bike to make the geometry work better for your body.

Either way, congrats on getting back into riding, and have fun as you keep pushing your distance farther!